Hippophage is an uncommon word that refers to a creature that primarily feeds on horses. While there are not many direct synonyms for this word, there are related terms that can be used to describe similar animals. Equinivorous is a preferable substitute as it encompasses any animal that feeds primarily on horses or any member of the equine family. Other possible options include equinophile, horse-eating, or simply carnivorous. However, these terms may not precisely capture the specific nature of a hippophage. It's worth noting that this is a rarely found word, and in most cases, a more common term could be used without losing the meaning intended.